Mark Egan

Over/under towed-streamer acquisition A method to extend seismic bandwidth to both higher and lower frequencies

Seismic resolution of towed marine-streamer data is affected by free-surface reflections. They strongly modulate the spectrum reducing energy at the so-called notch frequencies, fnu2009=u2009iv/2zu2009{iu2009=u20090,1,…,∞}, in which v is the water velocity and z is the source or receiver depth. As a result there is a very strong loss of useful low-frequency energy and usually a similar loss at higher frequencies. The ghost effect occurs both at the source and at the receiver. At the source, the upgoing part of the source wavefield is reflected off the free surface with inverted polarity before its delayed journey into the Earth. At the receiver, the upgoing part of the receiver wavefield is reflected off the free surface with inverted polarity before it travels down to the receiver again. The over/under acquisition method allows separation of the up- and downgoing wavefields at the source (or receiver) using a vertical pair of sources (or receivers) to determine wave direction. In this article, we present the results of two ...

Full-azimuth imaging using circular geometry acquisition

Wide-azimuth (WAZ) towed-streamer acquisition has improv-ed the quality and reliability of subsalt imaging in the Gulf of Mexico, where WAZ surveys have typically been ac-quired using 3–4 seismic vessels, each shooting in straight parallel lines. However, acquisition of WAZ data using a single vessel and a circular geometry offers several potential ad-vantages, both for operational efficiency and geophysical analysis of subsurface rock properties. Modeling exercises indicate that circular acquisition geo-metry can deliver a better range of azimuths and offsets than parallel WAZ geometry. A feasibility test in the Gulf of Mexico shows that it is possible to sail along circles while main-taining constant streamer separation and achieving very accurate receiver positioning. Single-sensor recording enables effective attenuation of the additional coherent noise introduced by shooting in a curve. Prestack depth-migrated data from the circular geometry test compare favorably with a parallel geometry WAZ data set...

Subsalt imaging The RAZ–WAZ experience

Is the salt winning? This was the question Paul Singer of Total posed at the subsalt workshop held at the Colorado School of Mines in the summer of 2005. This workshop was unique in that presenters from oil companies and contractors discussed their inability to successfully image under salt instead of the usual success stories generally related at most conferences.

Full Deghosting of OBC Data With Over/under Source Acquisition

The resolution of marine seismic data is affected by ghost and reverberations occurring in the water layer. A benefit of acquiring ocean-bottom cable data or node data is that the receiver-side ghosts and reverberations can be attenuated by combining pressure data, recorded with hydrophones, with velocity data, recorded with vertical geophones. Resolution of seismic data is also affected by the source ghosts. Acquisition of seismic data with over/under sources allows attenuating the source ghost and in this way to completely deghosting the ocean-bottom recorded data. In this paper, we present the results of an experiment where a 2D oceanbottom cable line was recorded with over/under sources.

Seismic Reservoir Characterization In Marcellus Shale

The Middle Devonian Marcellus shale that extends from Ohio and West Virginia, northeast into Maryland, Pennsylvania and New York, is believed to hold in excess of a thousand trillion ft of natural gas. High-quality surface seismic data and top-of-the-line processing are essential to characterize these reservoirs and the overburden formations for safe and cost-effective drilling. A workflow comprising data acquisition and processing to prestack seismic inversion and lithofacies classification for characterizing the shale reservoirs is presented. The key elements in this workflow are dense point-receiver data acquisition and processing in the point-receiver domain. A small data set acquired with a proprietary point-receiver system was available to demonstrate the benefits of this methodology. The data were in an area in New York, where the Marcellus formation is known to exist.

Shooting direction: A 3-D marine survey design issue

In the mid-1970s, 3-D marine survey design was primarily constrained by the ability to locate the position of the boat and streamer. Line spacings were, therefore, typically greater than 100 m. (Even if the navigation technology had been more accurate, it is likely that denser grids would not have been acquired because of the cost. This new method, 3-D, carried a price tag that was often considered uncomfortably high for a technology that was as yet unproven.) To minimize crossline aliasing effects, it was most prudent to record data in the dip direction.

Some aspects of survey design for wide-azimuth towed-streamer acquisition

Summary Wide-azimuth towed-streamer acquisition emerged in the last two years as a step change in marine acquisition technology in the Gulf of Mexico. This came about because the risky exploration and development of deepwater subsalt reservoirs required seismic data to have better illumination, higher signalto-noise ratio, and improved resolution. Early results from the initial programs confirm that the expected benefits can be obtained. However, although the survey design principles for land and OBC wideazimuth programs are well understood, the fact that the towed-streamer implementation is still in its infancy means that survey design principles in that case are still evolving. The objective of this paper is to discuss some of those principles.

A single-vessel method for wide-azimuth towed-streamer acquisition

Summary Wide-azimuth (WAZ) towed-streamer acquisition has been established as a successful method for exploration and development of the complex subsalt structures in the Gulf of Mexico. Wide-azimuth data acquired so far have proved to have better illumination, higher signal-to-noise ratio, and improved seismic resolution. The acquisition geometry used for WAZ surveys is a parallel geometry that employs multiple vessels. In this paper, we propose to acquire WAZ surveys with a single streamer vessel using a circular geometry and we describe the features of this new WAZ method. We also present the results of a feasibility test performed with circular geometry acquisition.

Acquisition Experiments For Improved Subsalt Imaging In Deepwater Gulf of Mexico

Summary Numerous deepwater, subsalt depth-imaging projects have shown the value of low frequencies. In this study, that observation was exploited through a series of acquisition experiments that were conducted over four fields in the Gulf of Mexico. The purpose was to see how much added benefit could be derived in processing and interpretation when rich low-frequency content was provided at the outset. The experimental methodologies consisted of an integration of steps that generated, preserved, and recorded more low-frequency signal while attacking the accompanying high-amplitude noise. In addition to giving attention to the low end of the spectrum, some of the experiments successfully focused on providing high frequencies as well.